This invention relates to an improved audio amplifier protection circuit which limits the power dissipation of the amplifier as necessary to protect the amplifier from overheating. Such circuits are used in radio receivers and other music reproduction systems using complex impedance speaker systems in limited space environments subject to large fluctuations in ambient temperature.
Music program material, with its wide dynamic range provided to complex speaker loads, is well known to produce a much lower average power output than a sine wave of the same peak level. It is inefficient and expensive to design an amplifier to dissipate any peak power level expected under any ambient temperature conditions conceivable. In addition, the large heat sinks required produce bulky amplifier packages. Customers are not overly receptive to large, expensive amplifiers, particularly in such environments as motor vehicles, where space is at a premium and prices are highly competitive. Therefore, such amplifiers are generally designed for normal expected use, at significant savings in bulk and cost, with temperature protection circuits provided to handle the rare circumstances in which, due to high ambient temperature and high audio power, temperature protection is required.
The typical temperature protection circuit removes bias from the amplifier when the junction temperature of the amplifier IC exceeds a predetermined safe level. If the circuit removes the bias quickly in great quantity, this results in complete loss of audio output and may lead to the amplifier cycling on and off, since the amplifier cools when shut down and heats when turned on. If the bias is removed in a more controlled manner, in order to try to reduce the power level gradually, the result in many amplifiers can be unstable operation or crossover distortion. In addition, circuits allowing full output power up to the desired shut down temperature are difficult to calibrate precisely. There are several variables involved; and the result can be a tolerance level as high as plus or minus 20 degrees C. in the shutdown temperature. There is need for a more precisely controllable temperature protection circuit for audio amplifiers that is less obtrusive to the listener.
An opportunity for just such a circuit exists in those audio amplifiers using a clipping detector to vary the gain of the input signal to reduce clipping distortion. The basis of this invention is the inclusion of the clipping detector in the temperature protection circuit by the inclusion of circuit elements reducing the allowable non-clipping voltage swing of the audio amplifier when the temperature thereof exceeds safe levels so as to induce clipping in the amplifier. The clipping detector thus causes a more precisely controllable reduction in gain and a consequent reduction in amplifier power for stable operation at a lower power level.